1. Field of the Invention
The present invention relates to a liquid ejection apparatus and an image forming apparatus using a liquid ejection apparatus, and more particularly, to air bubble removal technology suitable for removing an air bubble, which is a cause of an ejection defect, from a flow channel in such a liquid ejection apparatus as an inkjet head including a plurality of liquid droplet ejection ports (nozzles).
2. Description of the Related Art
In an inkjet type of recording apparatus, if an air bubble enters inside an ink flow channel, then an ejection defect occurs in that ink ceases to be ejected, or the ink ejection volume (the size of the dot formed by a droplet ejected onto a recording medium) or the droplet ejection position (direction of flight) becomes improper. In response to problems of this kind, in order to improve air bubble removal characteristics inside the ink flow channels, for example, Japanese Patent Application Publication No. 6-115087 discloses a structure in which the ends of the flow channels are formed to fine dimensions.
According to Japanese Patent Application Publication No. 6-115087, the cross-sectional area of an ink supply manifold which supplies ink to each of a plurality of ink supply channels is gradually reduced, the ink flow speed inside the manifold is maintained at or above a prescribed value, and thus the retention of air bubbles on the interior walls of the manifold is suppressed.
It is known that the removal characteristics of air bubbles are greatly dependent on the flow speed (m/s) in the flow channel. Here, the flow speed (m/s) is expressed as follows: “flow speed (m/s)=volume velocity (m3/s)/cross-sectional area of flow channel (m2)”. In other words, the reference to “raising the air bubble removal characteristics” described in Japanese Patent Application Publication No. 6-115087 means to increase the flow speed (m/s) by reducing the cross-sectional area of flow channel.
However, in recent inkjet recording apparatuses, due to demands for increased head length and compatibility with high-viscosity inks, and the like, situations have occurred where the cross-sectional area of the flow channel is inevitably required to increase, and this makes it difficult to remove the air bubbles on the basis of the flow speed.
Considering a case where a high-viscosity ink is used, since the flow channel resistance is directly proportional to the ink viscosity, then if the cross-sectional area of the flow channel is not increased sufficiently, it is not possible to keep the pressure loss inside the head (=flow channel resistance×volumetric speed) to within a specified value (for example, 800 Pa). If the pressure loss rises and exceeds the specified value, then it is difficult that the ink supply to the pressure chambers keeps up with demand, and eventually it becomes impossible to perform ejection.
Furthermore, considering a case where a long head is used, since the flow channel resistance is directly proportional to the length of the flow channel, then if the cross-sectional area of the flow channel is not increased sufficiently, it is not possible to keep the pressure loss inside the head (=flow channel resistance×volumetric speed) to within a specified value (for example, 800 Pa).
For these reasons, according to recent inkjet recording apparatuses, it has become difficult to sufficiently remove the air bubbles on the basis of the flow speed.